1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof. More particularly, the present invention relates to a thin film transistor (TFT) including an oxide or nitride semiconductor layer.
2. Discussion of the Background
In a conventional liquid crystal device or electroluminescence display device, a thin film transistor (TFT) may be used as a switching element to select a pixel or a part of pixel. A TFT typically includes an amorphous silicon or crystalline silicon semiconductor layer. A TFT using a crystalline silicon semiconductor layer has higher mobility, for example, 10 to 100 times more, than that of a TFT including an amorphous silicon semiconductor layer.
However, applying crystalline silicon complicates the fabrication process and decreases production yield. The heating temperature for crystallization of a silicon layer is about 550° C. or higher, so it may be difficult to use a flexible substrate made of plastic or something with a low melting point.
To replace an amorphous or crystalline silicon semiconductor layer, an oxide semiconductor layer, which may include zinc oxide (ZnO), has been developed in recent years. The oxide semiconductor layer has higher mobility, around 10 to 100 times higher than that of amorphous silicon (0.2 to 1.0 cm2/Vs) like the crystalline silicon semiconductor layer, and has much favorable properties like transparency and stability against external light.